Hi all,
I was reviewing unsaturated and saturated fatty acid esters (in biodiesel) and was wondering which ester would be more viscous?
My thinking...
Considering that unsaturated esters (due to double bonds) have a changed shape, they fit together less nicely, hence decreasing the dispersion forces between the molecules. Therefore, the molecules are less attracted to each other and unsaturated biodiesel esters are less viscous.
Is this correct? Thanks
You are correct! It is also important to consider the source of the components of certain biodiesels.
Plant oils often contain C=C double bonds, making them unsaturated, whilst animal fats such as tallow are known to contain only C-C single bonds, making them saturated.
The fatty acid's tail will determine most of its properties, including viscosity. Unsaturated fatty acids contain C=C double bonds, which often have geometric isomers i.e cis and trans formations (although trans formations are uncommon in natural fatty acids; they must be artifically produced). So, the cis arrangement around the double bond will affect the chains 3D shape because each carbon atom on a double bond is only connected to three other atoms (as opposed to the usual four of a saturated fatty acid) which will rearrange the atoms locations in 3D space, as they tend to localise the furthest possible from each other. As a result, the chains will take up more space, and there will be more space separating the neighboring chains because these chains simply cannot get any closer to each other. The increased distance that dispersion forces will have to travel weakens the forces, which will cause unsaturated biodiesel molecules to flow better, i.e have a lower viscosity.
Saturated fatty acids are straight chains, thus they will occupy less 3D space, meaning that neighboring chains can pack closer together, strengthening the dispersion forces as they will not have to travel as far. Thus, biodiesel sourced from saturated fats will not be able to flow as smoothly, i.e they have a higher viscosity.
We may also consider that neighboring biodiesel molecules will be able to form dipole-dipole bonds with the oxygen atoms in each ester group. As dipole-dipole forces are a lot stronger than dispersion forces, this will mean that all biodiesels are more viscous in general than petrodiesels.
We can see that the viscosity of biodiesel quite neatly relates back to it's source. Overlooking structure for a quick justification, animal fats are found as solid at room temperature, whilst plant oils are liquids at room temperature. Thus we can logically deduce that biodiesel molecules sourced from animal fats (saturated) will be more viscous than those sourced from plant oils (unsaturated).
Sorry if this was long, but hope it helps